Particulate magnetite obtained by aqueous solution reaction and the like is widely utilized as a powder material in various fields, especially of magnetic toners for MICR printers or dry electronic copiers and printers. Checks, securities, tickets, etc. are printed with identification marks called fonts in order to prevent forgery or alteration. The identification marks are printed in magnetic ink having a given amount of magnetic powder dispersed in a binder. The magnetic force of the magnetic powder is made use of to read the identification marks, fonts, with a special reader to give information, with which to distinguish between authenticity and inauthenticity of checks, etc. accurately. In applications to magnetic toners, magnetite particles are required to satisfy various general characteristics for development. The recent development of electrophotography has boosted rapid development particularly of copiers and printers using digital technologies, and the demands for magnetite particles with higher performance have ever been increasing.
For MICR application, toners capable of providing an increased print density and enhanced reading accuracy have been studied. Of development processes in electronic copying, development with a one-component developer, i.e., magnetite particles dispersed in a resin, is adapted widely. Today, magnetic toners used in electrostatic development, too, have been required to fulfill various characteristics to cope with the advanced performance of equipment in terms of size, precision, speed, and the like. In particular, a magnetic toner which causes little fog and achieves high resolution with excellent fine line reproducibility has been awaited.
It is required for an MICR toner to have an increased coercive force for increasing print density and improving reading accuracy. Magnetite particles as a powder material of magnetic toners are required to have a high residual magnetization and a high coercive force for suppressing fog. The related art is described in JP-A-59024.
JP-A-9-59024 refers to magnetite particles having high coercivity including those having an octahedral shape and those having a small particle size (or a large specific surface area).
However, the approach using octahedral particles encounters a limit on achievable coercivity, and the one relying on particle size reduction also has a limit because fine particles are liable to agglomerate heavily and difficult to disperse in toner preparation or inferior in blackness.
Among other proposals so far disclosed for increasing coercivity of magnetite particles are addition of a specific heavy metal, e.g., cobalt, heat treatment, compaction, and use of acicular particles with magnetic anisotropy. Taking into consideration the recent tendency to avoid use of environmentally unfriendly substances as well as economy, the state-of-the-art magnetite particles are still unsatisfactory in terms of higher coercivity, reduced environmental burden, and cost. For application to MICR toners, in particular, highly coercive acicular particles are usually used in a toner as a mixture with blackish isotropic particles, in which case the acicular particles are apt to be broken, or the resulting toner is poor in blackness.